1. Field of the Invention.
The invention relates to a surgical support or network for suspending or anchoring an implant in the bone or for reinforcing the bone cement used to anchor the components of prostheses.
2. DESCRIPTION OF THE PRIOR ART.
Usually, the components of joint replacements are anchored in the bone by means of a cold-polymerising polymethyl methacrylate (PMMA) or by locking them directly into the bone without bone cement. Recently, one has tried to anchor implant components more physiologically by means of pretensioned constructions (A. H. Huggler: Die Druckscheibenprothese im siebten Jahr ihrer klinischen Anwendung, in K. Draenert and A. Rutt (publishers); Histo-Morphologie des Bewegungsapparates 3, Art and Science, Munich, 1987).
The attempts to anchor the components of artificial joint replacements often fail because the forces cannot be transferred to the bone evenly. The result is that in cement-free components with a high modulus of elasticity, the presence of stress concentration (tension peak) in those areas which carry the main load leads to the consolidation of the bone, whereas the remaining bone segments atrophy as they are no longer made use of. In the case of cemented components, the bone cement fails to bear the very heavy loads permanently and therefore ruptures with the ultimate result that the components of the joint replacement become loose. of all the occurring forces, it is the shearing forces which have the most detrimental effect on the differentiation and development of the pluripotent mesenchyme, i.e. the mesenchyme tissue which ultimately produces the bone-forming cells known as osteoblasts.
There are constructions known from the field of architecture and statics with which one has attempted to use prestress in order to overcome these detrimental shearing forces, i.e. by reinforcing the concrete constructions with prestressed metal networks which take up the tensile strength (prestressed concrete). It is also possible to use free, pretensioned supports, such as light plane load-bearing structures, to convert tensile forces into pressure forces or compressive forces (Frei-Otto: Naturlichea Konstruktionen, Freie Verlagsanstalt, Stuttgrat, 1982). In the surgical locomotor system, too, several attempts have been made to adopt these principles, but they have been restricted to osteosynthesis, i.e. the use of screws in fracture treatment (Muller, Allgower and Willenegger, Manual der Osteosynthese, Springer Verlag, Heidelberg, Berlin, New York, 1969).
Constructions made of metal or carbon which reinforce the bone cement have already been used in the field of prosthetics, for example networks around the shafts of prostheses as described in U.S. Pat. No. 4 064 567. These networks do not form a close stocking and are not flexible. Due to their construction, they cannot not contact the bony bed of the prosthesis and thus their overall construction constitutes a prosthesis component in itself.
U.S. Pat. No. 4,457,028 describes multilayer, stocking-like reinforcements in the form of surgical networks which are partially absorbable and have a predetermined mesh size and filament thickness. These networks are excellent for reinforcing the cement stocking of a prosthesis, but are almost incapable of taking up the unfavourable shearing forces which occur in the interface between the prosthesis components and the bone. Furthermore, on account of the low affinity of the pure wire reinforcements described in DE-A-2 917 446 to the PMMA of the bone cement, they cannot achieve ideal contact with the cement matrix and can lead to filling effects which, in turn, represent predetermined breaking points. Hence when under heavy strain, the reinforcement can begin to wander and destroy the matrix. The absorbable materials according to U.S. Pat. No. 4,365,357 keep the bone in place; however, they may not be in a position to enable long-term load transmission from the prosthesis component to the bone.
Also, long-term studies have shown that conventional bone cement can excellently stand up to pressure in the body, and that for over 20 years, but it is not sufficiently resistant to tensile stress and bending stress and can collapse under the strain within a short period of time.